CuAAC-Based Click Chemistry in Self-Healing Polymers - Accounts of

Article Cite This: Acc. Chem. Res. 2017, 50, 2610-2620

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CuAAC-Based Click Chemistry in Self-Healing Polymers Diana Döhler, Philipp Michael, and Wolfgang H. Binder* Chair of Macromolecular Chemistry, Faculty of Natural Science II (Chemistry, Physics and Mathematics), Martin Luther University Halle-Wittenberg, Von-Danckelmann-Platz 4, D-06120 Halle (Saale), Germany CONSPECTUS: Click chemistry has emerged as a significant tool for materials science, organic chemistry, and bioscience. Based on the initial concept of Barry Sharpless in 2001, the copper(I)-catalyzed azide/alkyne cycloaddition (CuAAC) reaction has triggered a plethora of chemical concepts for linking molecules and building blocks under ambient conditions, forming the basis for applications in autonomous cross-linking materials. Self-healing systems on the other hand are often based on mild cross-linking chemistries that are able to react either autonomously or upon an external trigger. In the ideal case, self-healing takes place efficiently at low temperatures, independent of the substrate(s) used, by forming strong and stable networks, binding to the newly generated (cracked) interfaces to restore the original material properties. The use of the CuAAC in self-healing systems, most of all the careful design of copper-based catalysts linked to additives as well as the chemical diversity of substrates, has led to an enormous potential of applications of this singular reaction. The implementation of click-based strategies in self-healing systems therefore is highly attractive, as here chemical (and physical) concepts of molecular reactivity, molecular design, and even metal catalysis are connected to aspects of materials science. In this Account, we will show how CuAAC reactions of multivalent components can be used as a tool for self-healing materials, achieving cross-linking at low temperatures (exploiting concepts of autocatalysis or internal chelation within the bulk CuAAC and systematic optimization of the efficiency of the used Cu(I) catalysts). Encapsulation strategies to separate the click components by micro- and nanoencapsulation are required in this context. Consequently, the examples reported here describe chemical concepts to realize more efficient and faster click reactions in self-healing polymeric materials. Thus, enhanced chain diffusion in (hyper)branched polymers, autocatalysis, or internal chelation concepts enable efficient click cross-linking already at 5 °C with a simultaneously reduced amount of Cu(I) catalyst and increased reaction rates, culminating in the first reported self-healing system based on click cycloaddition reactions. Via tailor-made nanocarbon/Cu(I) catalysts we can further improve the click cross-linking reaction in view of efficiency and kinetics, leading to the generation of self-healing graphene-based epoxy nanocomposites. Additionally, we have designed special CuAAC click methods for chemical reporting and visualization systems based on the detection of ruptured capsules via a fluorogenic click reaction, which can be combined with CuAAC cross-linking reactions to obtain simultaneous stress detection and self-healing within polymeric materials. In a similar concept, we have prepared polymeric Cu(I)−biscarbene complexes to detect (mechanical) stress within self-healing polymeric materials via a triggered fluorogenic reaction, thus using a destructive force for a constructive chemical response.

1. INTRODUCTION After its discovery by Meldal and Sharpless1−3 the copper(I)catalyzed azide/alkyne cycloaddition (CuAAC) reaction has emerged as the most important linking reaction because of its ease of application as well as its width and simplicity,4−29 in this way brilliantly realizing the Sharpless click concept.30 The use of CuAAC-based click reactions in polymer science has been reviewed by us4−8 and others10,11 in the years 2007, 2008, and 2014. Basically, CuAAC overcomes the significant kinetic barrier of the purely thermal Huisgen 1,3-dipolar cycloaddition31,32 between azides and terminal alkynes by use of Cu(I) catalysis. 12,33−35 The current accepted mechanism5,11,35,36 explains the enormous rate increase by a factor of ∼107 in terms of the formation of a terminal copper(I) acetylide (Figure 1, I), followed by complexation of at least one more Cu(I) center to induce a bending of the azide (II) and the subsequent formation of six-membered cyclic intermediates by a binuclear Cu(I) complex (III), now templating the 1,3© 2017 American Chemical Society

dipolar cycloaddition (IV) to yield the 1,4-disubstituted 1,2,3triazole product (V). Several features make the CuAAC exceptional in comparison with many other reactions often mystified as click reactions:30,37 a high, often quantitative conversion; true substrate and solvent insensitivity at room temperature for thousands of substrates in nonaqueous and aqueous environments; the excellent stability of the participating reactive azides and alkynes combined with the simple (synthetic) availability of the functionalized raw materials; the ability to tune the reactivity by a simple choice of Cu(I) ligands, enabling an adjustable reaction rate;34,38,39 the observation of autocatalysis18,35 during the progress of the CuAAC, achieving a significant rate enhancement during the reaction, especially in, e.g., multivalent polymeric systems; the chance to couple the reaction with Received: July 26, 2017 Published: September 11, 2017 2610

DOI: 10.1021/acs.accounts.7b00371 Acc. Chem. Res. 2017, 50, 2610−2620

Article

Accounts of Chemical Research

Figure 1. Mechanism of the CuAAC enabling (A) mechanochemistry by copper(I) bis(N-heterocyclic carbene) catalysts, (B) fluorogenic click reactions, (C) room-temperature active self-healing materials, and stress-sensing systems including (D) autocatalytic and (E) internal chelation effects during the catalytic cycle.

triggered fluorescence (fluorogenic click chemistry) for applications in bioconjugation14 and (bio)analytical science29,40 and additionally to trigger the CuAAC by mechano-,20 photo-,41 or electrochemistry,42 achieving spatiotemporal control. To this end, the CuAAC can be considered as the only truly useful and widely accepted click reaction to chemically link large and voluminous molecules, with (bio)macromolecules being the prime species to this endeavor.7,8 The application of the CuAAC in self-healing materials is easily at hand, as chemical bonds have to be formed in a fast, efficient, and substrate-independent way under ambient conditions when materials need repair after being damaged. Therefore, one strategy for restoration is the use of multivalent liquid azides and alkynes in rapid cross-linking chemistries that are able to “glue” mechanical damage (e.g., microcracks within and on the material’s surface) efficiently. On the basis of White’s capsule-based healing concepts, the use of eff icient cross-linking reactions became evident, as exemplified by, e.g., ROMP-,43 epoxy-,44,45 and poly(siloxane)-based reactions.46 A

third, most important aspect in self-healing is the need for crosslinking at low temperatures: the ideal self-healing system naturally works autonomously, without external heating. Thus, the CuAAC is THE ideal chemistry to realize selfhealing materials at room temperature or even below. The combination of the CuAAC with self-healing is an ideal marriage, given that catalyst systems, self-healing agents, and the encapsulation of reactive compounds or the latency of the catalyst are well-chosen to realize a truly autonomous selfhealing material. Especially the incorporation of autocatalytic (Figure 1D) and internal chelation effects (Figure 1E) into the CuAAC enables its suitability for self-healing and stress-sensing applications (Figure 1B), significantly increasing reaction rates and lowering the required temperature. A plethora of different Cu(I) catalysts have been investigated,4−11,22−24,28,47−53 and the oxidation lability of many Cu(I) systems has been identified as a main drawback in the initial stage of the CuAAC for self-healing applications.4−11 Nowadays, creative ligand chemistry, together with 2611


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Figure 2. Selection of multivalent azides and alkynes as healing agents for a capsule-based self-healing approach, focusing on the influence of the architecture toward the optimization of healing agents (A−D) and copper catalysts (E−G).

architecture, and the number and density of functional groups (Figure 2A−D). Moreover, the applied Cu(I) catalysts have been optimized, starting with commercial Cu(I) salts,16−19,21,26 then using immobilized heterogeneous carbonaceous supported catalysts,22−24,27,28 and ending up with initially latent mechanocatalysts that are directly activated by the damage event itself20 (Figure 2E−G). The combination of CuAAC click chemistry with fluorogenic reactions is a novum in materials science20,25 that enables the location of mechanical damage within a self-healing material. The combination with carbon nanocomposites and mechanochemical activation allows catalysis beyond the conventional CuAAC, especially fast and highly ef f icient self-healing and stress sensing at lower temperatures.

the discovery of copper(I)−N-heterocyclic carbene (NHC) complexes,47−49 has significantly improved the applicability of many CuAAC systems, eg. for mechanochemistry (see Fgure 1A). An additional leap has been achieved with heterogeneous and supported Cu(I) systems, many of them based on copper(I) oxides, bound to, e.g., polymers or carbonaceous materials (carbon nanotubes, graphene, etc.).22−24,28,50,51 Overall there is an excellent interplay of click reactions and self-healing, as reflected in our own work during the past decade,16−26,54−61 showing a strong success in achieving autonomous as well as damage-triggered self-healing via balanced cross-linking chemistries. In this Account, we discuss the advantages of the CuAAC-based click chemistry displayed in self-healing materials at room temperature or below, not only enabling healing of damage under ambient conditions with a large variety of substrates and materials16−28,54,62 but also allowing damage reporting20,25 and the visualization of material failure triggering click reactions by mechanical force.20 We therefore systematically have used a large variety of potential healing agents16−19,21,26,54 varying in molecular weight,

2. CROSS-LINKING FOR AUTONOMOUS SELF-HEALING We first considered the CuAAC as a cross-linking principle in self-healing polymers in 2011,16 applied to a capsule-based selfhealing approach. Cross-linking chemistries based on the 2612


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60 °C, 91% or 107% of the tensile storage modulus, was recovered as a result of shear-induced click cross-linking (Figure 3C). As the CuAAC is often limited only by diffusion of the participating reactants, we expanded the variety of multivalent azides and alkynes to star-shaped as well as graft polymers with varying molecular weights, viscosities, and functional group densities. 18 Random poly(propargyl acrylate-co-n-butyl acrylate)s 15a−f with 2.8 to 16.7% alkyne content were designed with molecular weights ranging from 7.0 to 19.1 kg mol−1; star-shaped trivalent azide (9a−e, 5−30 kg mol−1) and alkyne (13, 6.3 kg mol−1) telechelic poly(isobutylene)s (PIBs) were synthesized, both with the rationale of increased molecular mobility. Subsequently, the CuAAC at room temperature was investigated via melt rheology (Figure 4A). Autocatalytic effects were observed, taking advantage of the formation of 1,2,3-triazole rings, which act as internal ligands, thus increasing the reaction rate significantly (Figure 4A, I).18,35 Once formed, the triazole rings are able to coordinate because of their close spatial proximity to the active Cu(I) center of the next ongoing CuAAC cycle (Figure 4A, II), favoring the formation of the copper(I) acetylide (Figure 1, I in the CuAAC mechanism). An acceleration of click cross-linking for 9a−e and 13 was observed at room temperature by a factor of 2.1 (9d, 30 kg mol−1) to 3.8 (9a, 5.5 kg mol−1) (Figure 4B). Decreasing the molecular weight in going from 9d to 9a accelerated the CuAAC further, an effect directly linked to the increased molecular mobility. Click cross-linking of 9 and 15 also revealed faster cross-linking for lower-molecular-weight polymers (e.g., 15b, 9.6 kg mol−1, to 15f, 19.1 kg mol−1; Figure 4C, half-solid symbols), which was further accelerated using polymers with a larger density of functional groups. Increasing the amount of alkyne groups from 2.8% alkyne monomer per chain for 15d to 16.7% for 15a enhanced the autocatalytic effects up to a factor of 4.3 (15a + 9e) during the cross-linking process (Figure 4C, solid symbols). Thus, the proper modeling of self-healing agents allowed the design of well-suited selfhealing materials. Further increasing the number of reactive end groups to obtain tougher self-healing materials culminated in hyperbranched azide- and alkyne-functionalized polymers 16 and 17 bearing up to nine functional groups.19 Because of the spherical shape of the polymers, click cross-linking proceeded significantly faster, highlighted by cross-linking times in the range of 30 to 50 min, additionally observing 3 to 10 times higher network densities. To further improve the cross-linking reaction for self-healing applications, we investigated chelation assistance within the CuAAC to allow fast and efficient network formation even below room temperature.26 External ligands, especially amines, have been described to protect the active copper(I) from oxidation and disproportionation and to additionally act as a base promoting the formation of the copper(I) acetylide.39,66 We prepared the trivalent picoline azide-functionalized polymer 11, known as a copper(I)-chelating moiety that acts as an internal ligand and therefore promotes the preorganization of the copper(I) acetylide (Figure 5).66 In the CuAAC reaction of trivalent picoline azide-functionalized polymer 11 and trivalent alkyne 13, complete crosslinking within 70 min at 10 °C using [Cu(PPh3)3]F as the catalyst was observed via melt rheology.26 Further kinetic investigations indeed revealed a doubling in the overall reaction rate in comparison with click cross-linking of alkyne 13 with

CuAAC reaction in this endeavor at this time were known only superficially:63,64 click-based polycondensations had been demonstrated to work as a bulk reaction system at elevated temperatures.33,65 To investigate the potential of the CuAAC toward selfhealing materials and to optimize the reactive healing agents, a series of multivalent, low-molecular-weight alkynes were systematically screened (Figure 2).16,17 Click cross-linking was followed via attenuated total internal reflection measurements and melt rheology, which showed a significantly faster reaction in the bulk state in comparison with model studies performed in solution,investigated via 1H NMR spectroscopy. During these investigations, alkyne 4 and azide 10 turned out to be the most efficient cross-linking agents in terms of gelation time and reaction temperature. Additionally, the applied Cu(I) catalysts were screened in view of their efficiency and operating temperature, which revealed [Cu(PPh3)3]Br to be the best catalyst for the cross-linking of 4 and 10 within 380 min at 40 °C.17 The most efficient healing agentsalkyne 4 and azide 10were encapsulated separately into poly(urea−formaldehyde) (PUF) microcapsules and embedded together with the best catalyst, [Cu(PPh3)3]Br, in a high-molecular-weight poly(isobutylene) matrix (Figure 3A). A photograph of the specimen before mechanical investigations clearly revealed the presence of unruptured capsules, while broken capsules were detected after application of a strain of ∼150% (Figure 3B). After the specimens were kept for 5 days at 25 °C or 3 days at

Figure 3. (A) Capsule-based self-healing approach using multivalent azides and alkynes. (B) Photographs of a self-healing specimen containing two types of PUF capsules filled with alkyne 4 and azide 10 as well as [Cu(I)(PPh3)3]Br/TBTA as catalytic system before and after strain. (C) Dynamic mechanical analysis (DMA) measurements before strain, directly after application of 150% strain, and after healing at 25 and 60 °C. Reproduced with permission from ref 16. Copyright 2011 Wiley-VCH. 2613


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Figure 4. (A) Melt rheology of click cross-linking of azide 9a−d with either alkyne 13 or 15a−f using [Cu(I)(PPh3)3]Br/TBTA catalyst at 20 °C and the proposed mechanism of autocatalysis. (B) Correlation of the rate constants k vs time for click cross-linking of 13 with 9a−d. (C) Correlation of the derivative of the reaction rate constant k versus time for click cross-linking of 15a−f with 9e. Adapted from ref 18. Copyright 2012 American Chemical Society.

azide-functionalized polymer 9 not containing the chelating picoline moiety.

3. SELF-HEALING NANOCOMPOSITES In the preparation of self-healing nanocomposites, carbonbased fillers have found important applications in polymer and materials science, as they can synergistically contribute to the mechanical properties of those composites, addressing the continuously growing quest to compensate for materials fatigue, especially after the addition of capsules filled with healing agents.67−69 For the development of self-healing epoxy nanocomposites containing heterogeneous Cu(I) catalysts, we aimed to develop easily scaled-up graphene-based Cu(I) catalysts with high stability against oxygen and outstanding catalytic activity toward click cross-linking approaches suitable for the development of capsule-based self-healing nanocomposites.22−24,27,28

Figure 5. Internal chelation assistance for click cross-linking of trivalent picoline azide-functionalized polymer 11 with alkyne 13 below room temperature.

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Figure 6. (A) Synthesis of thermally reduced graphene oxide Cu(I) nanoparticles (TRGO-Cu(I)2O, 18). (B) STEM-EDXS spectrum and (C) TEM image of 18. Adapted with permission from ref 22. Copyright 2014 The Royal Society of Chemistry.

Figure 7. (A) Dual-capsule-based self-healing epoxy nanocomposites containing two types of PUF capsules filled with azide 2 and alkyne 7 and different Cu(I) catalysts and their determined self-healing efficiencies. (B) Single-capsule-based self-healing epoxy nanocomposites containing PVF capsules filled with azide 2 and alkyne 7 directly dispersed in the matrix to investigate different homogeneous and heterogeneous Cu(I) catalysts and the determined self-healing efficiencies. Adapted with permission from ref 28. Copyright 2016 Wiley-VCH.

To this purpose, highly dispersed Cu(I) nanoparticles immobilized onto thermally reduced graphene oxide (TRGOCu(I)2O, 18) were generated as both catalyst and filler (Figure 6A),22 exhibiting a Cu2O particle size of roughly 25 nm. The desired oxidation state to perform a CuAAC click cross-linking reaction was confirmed by STEM−EDXS investigations displaying a Cu to O ratio of 2 to 1 (Figure 6B). Thus, an immobilized Cu(I) catalyst with a high loading of 7.6 × 10−4 mmolCu mgsample−1, excellent catalytic activity, and more than three times the recyclability without significant loss of activity was developed and used to prepare capsule-based self-healing epoxy nanocomposites. The addition of the azide/alkyne-loaded capsules influences the material strength: when probing the added capsules from 0 to 20 wt %, a continuous reduction in tensile strength and

Young’s modulus was observed with increasing capsule content, while optimal self-healing results were obtained for 15 wt % nanocapsules.28 Two types of PUF capsules (average size 400 nm), filled with either azide 2 or alkyne 7, together with the nanofiller 18 or a homogeneous Cu(I) catalyst, were embedded in the epoxy matrix.28 Within 3 h at 60 °C, a self-healing efficiency of 43% was observed for 18, while for the commercially available Cu(I) catalysts [Cu(I)(PPh3)3]F and [Cu(I)(PPh3)3]Br the self-healing efficiency was already 53 to 60% after 3 h. However, a significantly improved self-healing efficiency up to 91% was observed for 18 after 24 h at 60 °C, pointing at the enhanced stability of this catalyst (Figure 7A). Encouraged by these results, we developed a single-capsulebased self-healing approach28 in which alkyne 7 was directly dispersed in the epoxy matrix together with various homo- and 2615


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Figure 8. (A) Damage-responsive click cross-linking of azide 9 and alkyne 13 with azide-bearing coumarin-functionalized polymer 22 acting as a nonfluorescent probe showing fluorescence activation during the progress of the fluorogenic click reaction. (B) Changes in fluorescence intensity over time during the ongoing fluorogenic click reaction within a scratched and a control epoxy nanocomposite containing two types of PVF capsules filled with azides 9 and 22 and alkyne 13 as well as the [Cu(I)(PPh3)3]Br catalyst. Adapted with permission from ref 25. Copyright 2016 The Royal Society of Chemistry.

4. STRESS SENSING AND DAMAGE VISUALIZATION IN SELF-HEALING MATERIALS Besides self-healing, the visualization of damage and the sensing of stress within materials are important aspects in materials science that have received progressive attention in line with growing damage management concepts. Reporting systems that change color or fluorescence, such as spiropyranes, 70 anthracenes,71−73 and luminescent dioxetanes,74,75 have already found wide application in polymer science. We aimed to combine the CuAAC-capsule-based self-healing concept with a damage visualization strategy to sense stress while additionally monitoring the self-healing reaction.25 The concept is based on the encapsulation of a nonfluorescent azide-functionalized coumarin dye, which is released only after the capsules are broken by a damage event, consequently converting it into a fluorescent dye via a fluorogenic CuAAC click reaction (Figure 8A). The presence of a sufficiently mobile (liquid) form of the coumarin dye is important to enable a reaction with the alkyne after capsule rupture. We prepared the azide-bearing coumarinfunctionalized polymer 22 and added a 1 wt % loading of this polymer acting as a probe to an equimolar mixture of trivalent azide 9 and trivalent alkyne 13.25 Click reactions of these three components were first investigated in solution and showed an

heterogeneous copper catalysts, whereas only azide 2 was encapsulated within 100 nm small poly(vinyl formal) (PVF) capsules. After the sample was damaged, the recovery of the tensile storage modulus (E′) after 24 h with a healing efficiency of 70% and full recovery after 36 h were observed at room temperature using 18 as the catalyst (Figure 7B). In comparison, the homogeneous Cu(I) catalysts [Cu(PPh3)3]Br and [Cu(PPh3)3]F revealed a healing efficiency of only 45% after 24 h at room temperature, which further increased to 75% after 36 h and to 100% after 48 h (Figure 7B). Thus, 18 acted not only as the catalytically active species but also as a filler material, consequently counterbalancing the adverse effect of the reduced Young’s modulus and tensile strength related to the embedded nanocapsules. The excellent availability of the self-healing agents within the crack plane as well as the mechanical reinforcement underline the promising potential of this particular single-capsule-based self-healing approach not only for epoxy composites but also for silicon-based elastomers or poly(urethane)s, especially for aerospace engineering and the automotive industry. 2616


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Figure 9. (A) Mechanochemical activation of a latent copper(I) bis(N-heterocyclic carbene) catalyst by external force. The mechanocatalysts are embedded in a self-healing specimen together with multivalent azides and alkynes. (B) Structures of the PIB-, PS-, and PTHF-based biscarbene mechanocatalysts 19, 20, and 21. (C) Fluorogenic click reaction of phenylacetylene (23) with nonfluorescent azide-functionalized coumarin dye 24 after mechanochemical activation. (D) Mechanochemical activation of a latent copper(I)−NHC catalyst by compression and photographs of samples before and after compression showing increasing fluorescence intensity after 1, 2, 3, 10, and 20 cycles of compression. Adapted with permission from ref 20. Copyright 2015 Wiley-VCH.

emerging fluorescence via the fluorogenic click reaction linked to the activation of the former nonfluorescent probe 22 (Figure 8A). In bulk, a fluorescent cross-linked sample was obtained at room temperature, demonstrating the damage detection at the site of scratching (photograph in Figure 8B). This concept was incorporated into the already existing self-healing epoxy composite: similar to the described preparation of epoxy nanocomposites, 13 was encapsulated separately, while 9 and the nonfluorescent probe 22 were encapsulated together into PVF nanocapsules (average size 200 nm).25 Both types of nanocapsules were embedded in an epoxy matrix together with a dispersed Cu(I) catalyst. After the sample was scratched, an increase in fluorescence was observed over a time of 3000 min as the damaged nanocapsules released their contents, in turn triggering the fluorogenic click reaction (Figure 8B). Simultaneously, the self-healing reaction occurred in response to the applied force. A second strategy to prove damage within a material is the direct transfer of mechanical energy into a chemical response. Stimuli-responsive click reactions triggered by a defined external trigger,20,41,76,77 such as temperature, light, or force, directly linked to site-specific activation of a click reaction have already been described in materials science. Inspired by the idea to directly use a commonly destructive force to create a constructive chemical response, we designed mechanochemically triggered click catalysts specifically activated either by ultrasound or compression.20 The designed catalysts are based upon copper(I) bis(NHC) complexes that are centrally linked to various polymer chains (e.g., PIB, polystyrene (PS), polytetrahydrofuran (PTHF)) in order to investigate the influence of the attached polymer handle toward the activation behavior (Figure 9).20 These catalysts are inactive in their initial state, but can be activated for the CuAAC via an external mechanical force by cleavage of one of the shielding NHC ligands, which in turn generates the catalytically active monocarbene complexes.

One of the advantages of a latent catalytic system is the lack of encapsulating the self-healing agents, as the initial catalyst is not active because of the shielding NHC ligands, thus preventing preliminary reactions with the embedded Cu(I) catalyst before the damage event takes place (Figure 9A). Subjecting the latent catalysts in solution to ultrasound triggered a catalytic activity of 28% for the highly flexible PIB-based biscarbene complex 19 and of 52% for the stiffer PSbased biscarbene complex 20, while the PTHF-based catalyst 21 showed almost quantitative conversions toward a CuAAC click reaction of benzyl azide and phenylacetylene (23). Stress sensing in bulk can be accomplished via the abovedescribed fluorogenic click reaction. A mixture of 23 and the nonfluorescent azide-functionalized coumarin dye 24 was directly embedded together with the latent biscarbene catalyst into a high-molecular-weight polymer matrix (Figure 9C).20 Fluorescence spectroscopy after the specimen was damaged by compression via a hydraulic press revealed increasing fluorescence with increasing number of compression cycles by formation of the highly fluorescent product 25. The ongoing fluorogenic click reaction can be directly monitored and optically visualized by placing the compressed specimens under a UV lamp (Figure 9D). Quantifying the fluorescence intensity revealed conversions in the fluorogenic click reaction of up to 4%,20 increasing directly with increasing molecular weight of the attached polymer because of enhanced force transmission via the attached polymer chains. This nicely demonstrates the potential of the designed latent polymeric Cu(I)−biscarbene complexes as damage-sensing tools in materials science.

5. CONCLUSION Fast cross-linking and broad molecular and chemical diversity, combined with an excellent catalyst design and reactioncoupled fluorescence, make the CuAAC a highly attractive concept for self-healing materials. We have shown that aspects of the CuAAC can be transformed toward self-healing and 2617



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ACKNOWLEDGMENTS The authors acknowledge the German Science Foundation Projects DFG BI 1337/8-1 and BI 1137/8-2 within the framework of the SPP 1586 (“Design and Generic Principles of Self-Healing Materials”) and the SFB TRR 102/Project A3 for financial support. We also thank the present and former group members, colleagues, and collaborators.

damage-reporting polymers: linking basic mechanistic concepts of the CuAAC such as autocatalysis, internal chelation assistance, or the optimization of molecular diffusion via molecular design (e.g., multivalency or star-shaped, graft, or spherical architecture) with materials science has enabled the development of truly autonomous self-healing and damagesensing materials at room temperature and below. Graphene nanocomposites with self-healing properties can be fabricated by nanocarbon/Cu(I)O2 catalysts, further improving the click cross-linking reaction in view of efficiency and kinetics together with the material performance, finally reaching an effective selfhealing material at room temperature and below. Further extension to damage visualization as a stress-sensing tool established an additional feature into materials science, placing the CuAAC as the most versatile chemistry to combine selfhealing with simultaneous stress detection. The use of mechanochemically active, polymeric Cu(I)−biscarbene complexes enabled the detection of (mechanical) stress quantitatively within self-healing polymeric materials via fluorogenic reactions. Therefore, the implementation of click-based strategies for self-healing systems is a most prosperous area of materials science, as here chemical (and physical) concepts of molecular design, molecular reactivity, and even metal catalysis are directly connected to materials science.

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REFERENCES

(1) Meldal, M.; Tornøe, C. W. Peptidotriazoles: Copper(I)Catalyzed 1,3-Dipolar Cycloadditions on Solid-Phase. In Peptides: The Wave of the FutureProceedings of the Second International and the Seventeenth American Peptide Symposium; Houghten, R. A., Lebl, M., Eds.; Springer: Dordrecht, The Netherlands, 2001; pp 263−264. (2) Tornøe, C. W.; Christensen, C.; Meldal, M. Peptidotriazoles on Solid Phase: [1,2,3]-Triazoles by Regiospecific Copper(I)-Catalyzed 1,3-Dipolar Cycloadditions of Terminal Alkynes to Azides. J. Org. Chem. 2002, 67, 3057−3064. (3) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. A Stepwise Huisgen Cycloaddition Process: Copper(I)-Catalyzed Regioselective “Ligation” of Azides and Terminal Alkynes. Angew. Chem., Int. Ed. 2002, 41, 2596−2599. (4) Binder, W. H.; Sachsenhofer, R. “Click” Chemistry in Polymer and Materials Science. Macromol. Rapid Commun. 2007, 28, 15−54. (5) Binder, W. H.; Sachsenhofer, R. “Click” Chemistry in Polymer and Material Science: An Update. Macromol. Rapid Commun. 2008, 29, 952−981. (6) Binder, W. H.; Kluger, C. Azide/alkyne-“click” reactions: applications in material science and organic synthesis. Curr. Org. Chem. 2006, 10, 1791−1815. (7) Binder, W. H.; Sachenshofer, R. Click Chemistry on Supramolecular Materials. In Click Chemistry for Biotechnology and Materials Science; Lahann, J., Ed.; Wiley-Blackwell: Chichester, U.K., 2009; pp 119−175. (8) Binder, W. H.; Zirbs, R. “Click” Chemistry in Macromolecular Synthesis. In Encyclopedia of Polymer Science and Technology; John Wiley & Sons: Hoboken, NJ, 2014; Vol. 3, pp 186−230. (9) Demko, Z. P.; Sharpless, K. B. A Click Chemistry Approach to Tetrazoles by Huisgen 1,3-Dipolar Cycloaddition: Synthesis of 5Sulfonyl Tetrazoles from Azides and Sulfonyl Cyanides. Angew. Chem., Int. Ed. 2002, 41, 2110−2113. (10) Meldal, M.; Tornøe, C. W. Cu-Catalyzed Azide−Alkyne Cycloaddition. Chem. Rev. 2008, 108, 2952−3015. (11) Meldal, M. Polymer “Clicking” by CuAAC Reactions. Macromol. Rapid Commun. 2008, 29, 1016−1051. (12) Schoffelen, S.; Meldal, M. Alkyne−Azide Reactions. In Modern Alkyne Chemistry; Wiley-VCH: Weinheim, Germany, 2014; pp 113− 142. (13) Qin, A.; Lam, J. W. Y.; Tang, B. Z. Click polymerization. Chem. Soc. Rev. 2010, 39, 2522−2544. (14) Debets, M. F.; van Berkel, S. S.; Dommerholt, J.; Dirks, A. J.; Rutjes, F. P. J. T.; van Delft, F. L. Bioconjugation with Strained Alkenes and Alkynes. Acc. Chem. Res. 2011, 44, 805−815. (15) Le Droumaguet, C.; Wang, C.; Wang, Q. Fluorogenic click reaction. Chem. Soc. Rev. 2010, 39, 1233−1239. (16) Gragert, M.; Schunack, M.; Binder, W. H. Azide/Alkyne“Click”-Reactions of Encapsulated Reagents: Toward Self-Healing Materials. Macromol. Rapid Commun. 2011, 32, 419−425. (17) Schunack, M.; Gragert, M.; Döhler, D.; Michael, P.; Binder, W. H. Low-Temperature Cu(I)-Catalyzed “Click” Reactions for SelfHealing Polymers. Macromol. Chem. Phys. 2012, 213, 205−214. (18) Döhler, D.; Michael, P.; Binder, W. H. Autocatalysis in the Room Temperature Copper(I)-Catalyzed Alkyne-Azide “Click” Cycloaddition of Multivalent Poly(acrylate)s and Poly(isobutylene)s. Macromolecules 2012, 45, 3335−3345. (19) Dö h ler, D.; Zare, P.; Binder, W. H. Hyperbranched polyisobutylenes for self-healing polymers. Polym. Chem. 2014, 5, 992−1000.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Web: http:// www.macrochem.uni-halle.de/index-1.html. ORCID

Diana Döhler: 0000-0003-4996-1892 Philipp Michael: 0000-0003-0123-4581 Wolfgang H. Binder: 0000-0003-3834-5445 Notes

The authors declare no competing financial interest. Biographies Diana Döhler was born in Werdau (1988), and Philipp Michael was born in Halle (1986). They both studied chemistry at the MartinLuther-Universität Halle-Wittenberg (MLU) and completed their Master of Science degrees in 2011. Diana obtained her Ph.D. in 2015, and Philipp finished his Ph.D. in 2016. They worked on supramolecular polymers as well as on self-healing materials together with Professor Binder during their Master’s and Ph.D. studies dealing with the investigation and improvement of room-temperature-based click reactions and the development of mechanocatalysts for stress-sensing and self-healing applications. Currently they are habilitands in the research group of Professor Binder, and their current research interests include supramolecular chemistry, self-healing polymers, and nanocomposites as well as mechanochemistry. Wolfgang H. Binder is Full Professor of Macromolecular Chemistry at MLU since 2007. He was born (1969) and raised in Vienna, Austria, and studied chemistry at the University of Vienna (Ph.D. in organic chemistry (1995). After postdoctoral stays (1995−1997) with Prof. F. M. Menger (Emory University, Atlanta, GA) and Prof. J. Mulzer (University of Vienna), he completed his habilitation at the Vienna University of Technology (TU-Wien, 2004), where he was an Associate Professor from 2004 to 2007. His research interests include polymer synthesis, supramolecular chemistry, self-healing polymers, artificial membranes, and nanotechnology. For details, see www. macrochem.uni-halle.de. 2618


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Accounts of Chemical Research (20) Michael, P.; Binder, W. H. A Mechanochemically Triggered “Click” Catalyst. Angew. Chem., Int. Ed. 2015, 54, 13918−13922. (21) Vasiliu, S.; Kampe, B.; Theil, F.; Dietzek, B.; Döhler, D.; Michael, P.; Binder, W. H.; Popp, J. Insights into the Mechanism of Polymer Coating Self-Healing Using Raman Spectroscopy. Appl. Spectrosc. 2014, 68, 541−548. (22) Shaygan Nia, A.; Rana, S.; Döhler, D.; Noirfalise, X.; Belfiore, A.; Binder, W. H. Click chemistry promoted by graphene supported copper nanomaterials. Chem. Commun. 2014, 50, 15374−15377. (23) Shaygan Nia, A.; Rana, S.; Döhler, D.; Osim, W.; Binder, W. H. Nanocomposites via a direct graphene-promoted “click”-reaction. Polymer 2015, 79, 21−28. (24) Shaygan Nia, A.; Rana, S.; Döhler, D.; Jirsa, F.; Meister, A.; Guadagno, L.; Koslowski, E.; Bron, M.; Binder, W. H. CarbonSupported Copper Nanomaterials: Recyclable Catalysts for Huisgen [3 + 2] Cycloaddition Reactions. Chem. - Eur. J. 2015, 21, 10763−10770. (25) Döhler, D.; Rana, S.; Rupp, H.; Bergmann, H.; Behzadi, S.; Crespy, D.; Binder, W. H. Qualitative sensing of mechanical damage by a fluorogenic “click” reaction. Chem. Commun. 2016, 52, 11076− 11079. (26) Neumann, S.; Döhler, D.; Ströhl, D.; Binder, W. H. Chelationassisted CuAAC in star-shaped polymers enables fast self-healing at low temperatures. Polym. Chem. 2016, 7, 2342−2351. (27) Raimondo, M.; De Nicola, F.; Volponi, R.; Binder, W. H.; Michael, P.; Russo, S.; Guadagno, L. Self-repairing CFRPs targeted towards structural aerospace applications. IJSI 2016, 7, 656−670. (28) Rana, S.; Döhler, D.; Nia, A. S.; Nasir, M.; Beiner, M.; Binder, W. H. “Click”-Triggered Self-Healing Graphene Nanocomposites. Macromol. Rapid Commun. 2016, 37, 1715−1722. (29) Hvilsted, S. Facile design of biomaterials by “click” chemistry. Polym. Int. 2012, 61, 485−494. (30) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Click Chemistry: Diverse Chemical Function from a Few Good Reactions. Angew. Chem., Int. Ed. 2001, 40, 2004−2021. (31) Huisgen, R. Kinetics and reaction mechanisms: selected examples from the experience of forty years. Pure Appl. Chem. 1989, 61, 613−628. (32) Huisgen, R. 1.3-Dipolare Cycloadditionen Rückschau und Ausblick. Angew. Chem. 1963, 75, 604−637. (33) Sheng, X.; Mauldin, T. C.; Kessler, M. R. Kinetics of Bulk Azide/Alkyne “Click” Polymerization. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 4093−4102. (34) Rodionov, V. O.; Presolski, S. I.; Díaz Díaz, D.; Fokin, V. V.; Finn, M. G. Ligand-Accelerated Cu-Catalyzed Azide−Alkyne Cycloaddition: A Mechanistic Report. J. Am. Chem. Soc. 2007, 129, 12705− 12712. (35) Rodionov, V. O.; Fokin, V. V.; Finn, M. G. Mechanism of the Ligand-Free CuI-Catalyzed Azide−Alkyne Cycloaddition Reaction. Angew. Chem., Int. Ed. 2005, 44, 2210−2215. (36) Worrell, B. T.; Malik, J. A.; Fokin, V. V. Direct Evidence of a Dinuclear Copper Intermediate in Cu(I)-Catalyzed Azide-Alkyne Cycloadditions. Science 2013, 340, 457. (37) Barner-Kowollik, C.; Du Prez, F. E.; Espeel, P.; Hawker, C. J.; Junkers, T.; Schlaad, H.; Van Camp, W. Clicking” Polymers or Just Efficient Linking: What Is the Difference? Angew. Chem., Int. Ed. 2011, 50, 60−62. (38) Rodionov, V. O.; Presolski, S. I.; Gardinier, S.; Lim, Y.-H.; Finn, M. G. Benzimidazole and Related Ligands for Cu-Catalyzed Azide− Alkyne Cycloaddition. J. Am. Chem. Soc. 2007, 129, 12696−12704. (39) Presolski, S. I.; Hong, V.; Cho, S.-H.; Finn, M. G. Tailored Ligand Acceleration of the Cu-Catalyzed Azide−Alkyne Cycloaddition Reaction: Practical and Mechanistic Implications. J. Am. Chem. Soc. 2010, 132, 14570−14576. (40) Hein, C. D.; Liu, X.-M.; Wang, D. Click Chemistry, A Powerful Tool for Pharmaceutical Sciences. Pharm. Res. 2008, 25, 2216−2230. (41) Tasdelen, M. A.; Yagci, Y. Light-Induced Click Reactions. Angew. Chem., Int. Ed. 2013, 52, 5930−5938.

(42) Hong, V.; Udit, A. K.; Evans, R. A.; Finn, M. G. Electrochemically Protected Copper(I)-Catalyzed Azide−Alkyne Cycloaddition. ChemBioChem 2008, 9, 1481−1486. (43) White, S. R.; Sottos, N. R.; Geubelle, P. H.; Moore, J. S.; Kessler, M. R.; Sriram, S. R.; Brown, E. N.; Viswanathan, S. Autonomic healing of polymer composites. Nature 2001, 409, 794−797. (44) Jin, H.; Mangun, C. L.; Stradley, D. S.; Moore, J. S.; Sottos, N. R.; White, S. R. Self-healing thermoset using encapsulated epoxyamine healing chemistry. Polymer 2012, 53, 581−587. (45) Yuan, Y. C.; Rong, M. Z.; Zhang, M. Q.; Chen, J.; Yang, G. C.; Li, X. M. Self-Healing Polymeric Materials Using Epoxy/Mercaptan as the Healant. Macromolecules 2008, 41, 5197−5202. (46) Cho, S. H.; Andersson, H. M.; White, S. R.; Sottos, N. R.; Braun, P. V. Polydimethylsiloxane-Based Self-Healing Materials. Adv. Mater. 2006, 18, 997−1000. (47) Marion, N.; Díez-González, S.; Nolan, S. P. N-Heterocyclic Carbenes as Organocatalysts. Angew. Chem., Int. Ed. 2007, 46, 2988− 3000. (48) Díez-González, S.; Nolan, S. P. [(NHC)2Cu]X Complexes as Efficient Catalysts for Azide−Alkyne Click Chemistry at Low Catalyst Loadings. Angew. Chem., Int. Ed. 2008, 47, 8881−8884. (49) Bantreil, X.; Broggi, J.; Nolan, S. P. N-Heterocyclic carbene containing complexes in catalysis. Annu. Rep. Prog. Chem., Sect. B: Org. Chem. 2009, 105, 232−263. (50) Hein, J. E.; Fokin, V. V. Copper-catalyzed azide-alkyne cycloaddition (CuAAC) and beyond: new reactivity of copper(I) acetylides. Chem. Soc. Rev. 2010, 39, 1302−1315. (51) Chassaing, S.; Beneteau, V.; Pale, P. When CuAAC “Click Chemistry” goes heterogeneous. Catal. Sci. Technol. 2016, 6, 923−957. (52) Fu, Q.; Xie, K.; Tan, S.; Ren, J. M.; Zhao, Q.; Webley, P. A.; Qiao, G. G. The use of reduced copper metal-organic frameworks to facilitate CuAAC click chemistry. Chem. Commun. 2016, 52, 12226− 12229. (53) Deraedt, C.; Pinaud, N.; Astruc, D. Recyclable Catalytic Dendrimer Nanoreactor for Part-Per-Million CuI Catalysis of “Click” Chemistry in Water. J. Am. Chem. Soc. 2014, 136, 12092−12098. (54) Zhao, Y.; Döhler, D.; Lv, L.-P.; Binder, W. H.; Landfester, K.; Crespy, D. Facile Phase-Separation Approach to Encapsulate Functionalized Polymers in Core−Shell Nanoparticles. Macromol. Chem. Phys. 2014, 215, 198−204. (55) Michael, P.; Döhler, D.; Binder, W. H. Improving Autonomous Self Healing via Combined Chemical/Physical Principles. Polymer 2015, 69, 216−227. (56) Döhler, D.; Binder, W. Von der Muschel zum Autolack. Nachr. Chem. 2016, 64, 836−841. (57) Dö hler, D.; Michael, P.; Neumann, S.; Binder, W. H. Selbstheilende Polymere. Chem. Unserer Zeit 2016, 50, 90−101. (58) Döhler, D.; Michael, P.; Binder, W. H. Principles of Self Healing Polymers. In Self Healing Polymers: From Principles to Application; Binder, W. H., Ed.; Wiley-VCH: Weinheim, Germany, 2013; pp 7−60. (59) Shaygan Nia, A.; Binder, W. H. Graphene as initiator/catalyst in polymerization chemistry. Prog. Polym. Sci. 2017, 67, 48−76. (60) Li, N.; Binder, W. H. Click-chemistry for nanoparticlemodification. J. Mater. Chem. 2011, 21, 16717−16734. (61) Binder, W. H.; Kluger, C. Combining Ring-Opening Metathesis Polymerization (ROMP) with Sharpless-Type “Click” Reactions: An Easy Method for the Preparation of Side Chain Functionalized Poly(oxynorbornenes). Macromolecules 2004, 37, 9321−9330. (62) Herbst, F.; Döhler, D.; Michael, P.; Binder, W. H. Self-Healing Polymers via Supramolecular Forces. Macromol. Rapid Commun. 2013, 34, 203−220. (63) Liu, Y.; Díaz, D. D.; Accurso, A. A.; Sharpless, K. B.; Fokin, V. V.; Finn, M. G. Click Chemistry in Materials Synthesis. III. MetalAdhesive Polymers from Cu(I)-Catalyzed Azide-Alkyne Cycloaddition. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 5182−5189. (64) Díaz, D. D.; Punna, S.; Holzer, P.; McPherson, A. K.; Sharpless, K. B.; Fokin, V. V.; Finn, M. G. Click chemistry in materials synthesis. 1. Adhesive polymers from copper-catalyzed azide-alkyne cycloaddition. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 4392−4403. 2619


Article

Accounts of Chemical Research (65) Sheng, X.; Rock, D. M.; Mauldin, T. C.; Kessler, M. R. Evaluation of different catalyst systems for bulk polymerization through “click” chemistry. Polymer 2011, 52, 4435−4441. (66) Bevilacqua, V.; King, M.; Chaumontet, M.; Nothisen, M.; Gabillet, S.; Buisson, D.; Puente, C.; Wagner, A.; Taran, F. CopperChelating Azides for Efficient Click Conjugation Reactions in Complex Media. Angew. Chem., Int. Ed. 2014, 53, 5872−5876. (67) Domun, N.; Hadavinia, H.; Zhang, T.; Sainsbury, T.; Liaghat, G. H.; Vahid, S. Improving the fracture toughness and the strength of epoxy using nanomaterials - a review of the current status. Nanoscale 2015, 7, 10294−10329. (68) Ghadami, F.; Dadfar, M. R.; Kazazi, M. Hot-cured epoxynanoparticulate-filled nanocomposites: Fracture toughness behavior. Eng. Fract. Mech. 2016, 162, 193−200. (69) Brown, E. N.; Sottos, N. R.; White, S. R. Fracture testing of a self-healing polymer composite. Exp. Mech. 2002, 42, 372−379. (70) Davis, D. A.; Hamilton, A.; Yang, J.; Cremar, L. D.; Van Gough, D.; Potisek, S. L.; Ong, M. T.; Braun, P. V.; Martinez, T. J.; White, S. R.; Moore, J. S.; Sottos, N. R. Force-induced activation of covalent bonds in mechanoresponsive polymeric materials. Nature 2009, 459, 68−72. (71) Gostl, R.; Sijbesma, R. P. π-extended anthracenes as sensitive probes for mechanical stress. Chem. Sci. 2016, 7, 370−375. (72) Song, Y.-K.; Lee, K.-H.; Hong, W.-S.; Cho, S.-Y.; Yu, H.-C.; Chung, C.-M. Fluorescence sensing of microcracks based on cycloreversion of a dimeric anthracene moiety. J. Mater. Chem. 2012, 22, 1380−1386. (73) Radl, S.; Kreimer, M.; Griesser, T.; Oesterreicher, A.; Moser, A.; Kern, W.; Schlögl, S. New strategies towards reversible and mendable epoxy based materials employing [4πs + 4πs] photocycloaddition and thermal cycloreversion of pendant anthracene groups. Polymer 2015, 80, 76−87. (74) Chen, Y.; Spiering, A. J. H.; Karthikeyan, S.; Peters, G. W. M.; Meijer, E. W.; Sijbesma, R. P. Mechanically induced chemiluminescence from polymers incorporating a 1,2-dioxetane unit in the main chain. Nat. Chem. 2012, 4, 559−562. (75) Clough, J. M.; Balan, A.; van Daal, T. L. J.; Sijbesma, R. P. Probing Force with Mechanobase-Induced Chemiluminescence. Angew. Chem., Int. Ed. 2016, 55, 1445−1449. (76) Harmand, L.; Cadet, S.; Kauffmann, B.; Scarpantonio, L.; Batat, P.; Jonusauskas, G.; McClenaghan, N. D.; Lastécouères, D.; Vincent, J.-M. Copper Catalyst Activation Driven by Photoinduced Electron Transfer: A Prototype Photolatent Click Catalyst. Angew. Chem., Int. Ed. 2012, 51, 7137−7141. (77) Rinaldi, L.; Martina, K.; Baricco, F.; Rotolo, L.; Cravotto, G. Solvent-Free Copper-Catalyzed Azide-Alkyne Cycloaddition under Mechanochemical Activation. Molecules 2015, 20, 2837−2849.

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